1. Technical Field
This invention relates generally to brakes and, more particularly, to a method of designing, prototyping and manufacturing disc brake pads.
2. Related Art
Disc brake squeal is a significant problem associated with disc brake systems, and has been a major contributor to noise related brake warranty claims. It is well known that disc brake squeal is primarily a phenomenon of instability due to friction induced vibration. Numerous studies and reviews have been conducted on friction induced vibration in general, and disc brake squeal phenomenon in specific, which have revealed that a squeal event involves two prominent mechanisms, negative damping and mode locking. Negative damping refers to the negative slope of damping versus velocity in the brake system, which essentially creates a self-excited oscillation system that provides energy to sustain squeal. Mode locking is the phenomenon that resonance modes of different brake components (pads, rotor, caliper, and anchor bracket) with close natural frequencies and similar mode shapes come together under the system coupling condition and behave as a single system resonant mode.
Some manufacturers use noise insulators, such as various shims and molded noise insulators on the side of the backing plate opposite the friction pad material to reduce unwanted brake noise. Modification of other brake system components, such as the rotors, calipers and anchor brackets have also been used to reduce unwanted noise.
For high frequency squeals the influence of the caliper and anchor bracket are diminished above approximately 5 kHz, and that noise insulators lose effectiveness above approximately 12 kHz. Modification of the rotor rarely provides significant reduction in high frequency squeals. Applicants believe that rotors have many resonant modes that fall into frequency ranges of interest. Modification of the rotor rarely provides significant reduction in high frequency squeal because rotors have many resonant modes that fall into the frequency range of interest. Redesigning rotors also involve higher cost and longer lead times.
Pad shape modification has been an effective means for reducing brake squeal, particularly high frequency squeal, and to reduce disc brake squeal some manufacturers have modified the shape of the friction pad material. Some of these modifications have included various chamfers on the ends of the friction pads, various transverse slots in the friction pads and combinations of end chamfers and transverse slots. Modifications to the shape of the friction material can be implemented quickly with relatively lower costs than modification of other brake components, and can be utilized both by original equipment manufacturers and automotive aftermarket suppliers. Also, since brake pads are wear parts, modifications to the pad shape can improve the performance of brake systems already in use.
The problem with pad shape modifications is that these modifications by themselves do not always achieve an acceptable amount of reduction of the brake squeal noise.
Additionally, finite element models with various degrees of sophistication have been developed for brake squeal analysis. It has been reported that a comprehensive model including pads, rotor, caliper, and knuckle assembly based on complex eigenvalue analysis has the ability of predicting low frequency squeal, which is related to more components in the brake system. However, building a comprehensive brake system model requires detailed information of all components involved, which might not be available during early stages of the design. Also, assumptions on different interface conditions, (i.e., support stiffness and damping) must be made in order to couple various components together in such models. Furthermore, friction behavior between the rotor and pads must be assumed in order to capture the negative damping mechanism. Validation of these models require at least the knowledge of the squeal frequencies and amplitudes, which again might not be available. An alternative model reported in a paper entitled “Reducing High Frequency Disc Brake Pad Squeal by Pad Shape Optimization” published as SAE Paper No. 2000-01-0444″, including only the pads and rotor with caliper support stiffness based on frequency response analysis has been reported to substantially reduce the effort of building and executing a squeal analysis model while maintaining the capability of capturing the mode locking mechanism. However, this model still requires inputs from certain brake system components other than the brake pads, such as from the rotor and caliper, which makes it difficult to use in situations where large number of brake pad designs need to be evaluated, or where the information mentioned is simply not available.
While predictive models of the types described above have been reported, there exists a need for improved methods to analyze and predict this brake squeal, particularly high frequency brake squeal, for use in conjunction with the design, prototyping and manufacture disc brake pads.